A number of naturally occurring peptides, including oxytocin, secretin, apamin, and several releasing hormones from the brain, are peptide amides, that is, they contain an amide function at the carboxyl terminal. The synthesis of C-terminal peptide amides can be problematical because of the acid-sensitivity of some of these peptides, and the tendency of some amino acid residues, such as tryptophan, to become alkylated under the conditions used in most synthesis methods. The synthesis of peptide amides by solid-phase synthesis has most commonly involved releasing the peptide amide from the solid phase using either a strong acid (e.g., anhydrous hydrogen fluoride) or a strong base (e.g., ammonia) for final cleavage. For example, ammonolysis of benzyl and other ester anchoring linkages has been used, or benzhydrylamine support resins have been applied in conjunction with protection strategies that use anhydrous hydrogen fluoride (HF) for the final cleavage of the peptide from the support. Such harsh conditions can cause undesirable side reactions. G. Barany and R. B. Merrifield, In: The Peptides, E. Gross and J. Meienhofer (eds.), Vol. 2, pp. 1-284, Academic Press, New York (1979).
Several drawbacks to these methods exist. For example, ammonolysis of sterically hindered amino acids, such as valine, proceeds very slowly, and is even further retarded with the increasing length of the peptide chain. In addition, this technique is limited to peptides lacking C.sup..omega. -benzyl-protected aspartate or glutamate residues, which would be vulnerable to attack by ammonia. Finally, prolonged treatment with ammonia leads to partial racemization of Cys(Acm) residues. Numerous difficulties have been documented as well, which accompany cleavage with strong acids, such as HF.
Solutions which have been proposed for these problems include milder conditions and orthogonal protection schemes. An "orthogonal" system is defined as one using two or more independent classes of protecting groups which are removed by different chemical mechanisms. The most flexible approach for the solid-phase synthesis of peptide amides appears to involve the use in orthogonal systems of handles which incorporate a precursor of the amide function. These handles are coupled onto amine-functionalized solid supports and serve as a starting point for peptide chain elongation. For example, handles which are useful for anchoring tert-butyloxycarbonyl (Boc)-protected amino acids to solid supports are described by Gaehde and Matsueda in Int. J. Peptide Protein Res., l 18:451 (1981). Albericio and Barany describe handles for use with Fmoc-protected amino acids. F. Albericio and G. Barany, Int. J. Peptide Protein Res., 30:206-216 (1987); Albericio et al., J. Organic Chemistry, 55:3730-3743 (1990). However, the synthesis and/or cleavage conditions used in the referenced methods are too harsh for some peptide amides. A handle which allows acid-sensitive peptide amides to be efficiently produced and cleaved from the support under mild conditions would be valuable.